In the manufacture of a semiconductor, a layer of photoresist may be applied directly to the surface of a substrate or to layer(s) already on the surface of the substrate. A portion of the photoresist is then exposed to u.v. light to form a pattern on the photoresist. Then a solvent is applied to remove either the exposed or unexposed photoresist as desired.
Light scattering or interference caused by light reflecting off the substrate, for example, causes a dimension change in the exposed region of the photoresist layer. In other words, the desired dimensions of the exposed photoresist layer are altered by the light reflecting off the surface of the substrate. In some semiconductors small variations in the desired dimensions may be acceptable. However, there is usually a point in which no further variation can be tolerated because such variation alters the desired characteristics and performance of the semiconductor. The point at which no further variation can be tolerated is called the critical dimension ("CD").
In lithography systems, thin film interference effects result in less controllability of CD and less accurate alignment. These problems are more significant for illumination light having a short wavelength, e.g., 248 nm of a KrF excimer laser and 193 nm of an ArF excimer laser.
Two solutions to suppress thin film interference and reduce light scattering are bottom antireflective coatings (BARLs) and top antireflective coatings (TARs). A BARL is usually applied to a substrate and baked prior to applying the photoresist layer. After the photoresist layer is applied, patterns are resolved in the photoresist by DUV light in the case of the practical BARL process. Then, the photoresist patterns are transferred to both the BARL and the substrate by reactive ion etching (RIE) continuously. After the pattern transfer, both the desired photoresist and BARL portions are removed. Accordingly, the BARL must be more effective to reduce light scattering or interference caused by light reflecting off the substrate due to the higher light absorption. However, there are few available BARLs to completely suppress the light scattering or interference without reducing alignment light.
A TAR is usually spun onto the applied photoresist and baked. Patterns are resolved in both the TAR and the photoresist by DUV light simultaneously. The TAR portions are usually removed by the solvent (developer) completely, in the case of the practical TAR process. Then, the photoresist patterns are transferred to the substrate to RIE. After the pattern transfer, the desired photoresist portions are removed. Accordingly, the TAR must be transparent to some extent to let both incident light and light reflecting off the substrate go off. However, it has been difficult to obtain a TAR layer which has a suitable index of refraction for the wavelength of the illumination light and no contamination on the interface between the underlayer resist and the TAR itself.
In advanced lithography, it is well known that a reduction of photoresist thickness induces imagings of decreased k.sub.1 factor and increased k.sub.2 factor in conventional Rayleigh equations simultaneously without changing any wavelengths of the illumination light and numerical apertures of the optics. On the other hand, it causes more serious thin film interference effects and requires a higher etch selectivity ratio. In order to restore such concomitant problems using a conventional single layer resist technique, either of the photoresist or the TAR is required to have higher etch resistance than previously expected.